The invention relates to a method for the recovery of sediments from the bottom of the sea by means of a freely suspended suction pipe provided on one end with a suction mouth.
The invention further relates to apparatus for carrying out the method, comprising a floating body from which the suction pipe is suspended which carries at its lower end the suction head having attached thereto loosening means to dislodge the sediment to be recovered.
A prior art conveyor apparatus is known from German patent specification DE-OS No. 2 707 899, which includes a conveyor pipe the lower end of which is movable and in tightly fitting circumferential relationship with a cylindrical structure. Disposed between the cylinder and the end of the conveyor pipe is a drive means to reciprocate the two parts in an opposed motion. This reciprocating motion not only serves to produce a pumping action, but also produces high frequency vibrations. Such vibrations are intended to assist in the penetration of the mud to be conveyed and to prevent the creation of channels therein. This object, however, is achieved only partially achieved in actual practice.
The same disadvantage has been found in the conveyor apparatus according to the earlier German patent specification No. P 28 41 203.5 in which vibrations are generated in a similar fashion as in the afore mentioned well known device to loosen bottom formations. The vibratory movements are performed by a vibratory screen which may be in the shape of a cone pointing downward so that the direction of vibration is vertical. A device of this type is not capable of successfully loosening and dislodging relatively compact formations of mud-like consistency from the sea bottom as they occur, for instance, in the Red Sea at great depths. In any event, a device of this type is not capable of adequately loosening and dislodging sediment at any great depths in the sediment layers, but only near the less compacted surface of the sediment where the material is of a sufficient fluidity.
It is the object of the present invention to provide a method for the recovery of sediments by means of a freely suspended suction pipe by which sediments having the consistency of compacted mud can be recovered.
The object of the present invention is achieved by a method in which the suction means in the form of a suction mouth is successively slowly lowered into the sediment to be recovered, is raised up again to a height at which it is freely laterally movable, is laterally moved a predetermined distance, is lowered again, and so on, in a repeating cycle. The predetermined path of lateral movement is such that a lateral sliding of the suction mouth down into the depression formed in the sediment by the previous work cycle is avoided.
This teaching is based on the experience that vibrators do not produce a sufficiently effective loosening action. Moreover, the loosening effect of the well known prior suction heads provided with vibrators is limited to the area immediately surrounding the screen or to top layers of the sediment which are of a sufficiently low viscosity. Furthermore, the invention takes into account the fact that a lateral feed thrust, for instance in surface regions of the sediment which are of a satisfactory viscosity, will meet with difficulties at greater depths, such as for example 2000 meters. With this in mind, the method of the invention provides for the recovery operation or the forward thrust, respectively, to take place principally in the downward direction, by slowly lowering the suction mouth into the sediment, with the speed being so adjusted that the sediment portions in front of the suction mouth will be dislodged. In view of the high flow speeds in the region of the rims of the suction mouth, a loosening of even relatively compact mud-like formations is possible. Upon completion of the downwardly directed stripping action, which forms a more or less cylindrical to conical depression in the sediment, the method of the invention does not even attempt to continue the recovery operation in a sideways direction. Rather, the suction mouth is raised and is laterally moved a distance such that during the subsequent slow lowering of the suction mouth, any guiding forces in the sediment which may have been generated by the previous work cycle and which would cause the suction mouth to slide down into the previously formed depression, are rendered ineffective. Thus, it is insured that the suction mouth again is able to penetrate vertically into the sediment at the new location adjacent the previously worked depression to, thereby, form a new depression in the sediment to be recovered. In this manner, it is possible to expedite the penetration of the sediment by the suction mouth by using mass forces. By concentrating large masses in the suction head, considerable forces are realized enabling the suction head to penetrate also into relatively solid sediments and to loosen the material.
The lateral movement of the suction mouth attached to the suction pipe, which is freely suspended during the lateral movement, does not require a corresponding movement of the upper end of the suction pipe. Instead, it is sufficient to progressively move the upper end of the suction pipe sideways on the surface of the water, at a speed which corresponds to the mean lateral velocity of the suction mouth. During the lowering of the suction assembly into the sediment, the lower end of the suction pipe is guided in the respective depression so that small lateral forces due to the slightly sloping direction of the suction pipe caused by its steadily being advanced have no effect. When the suction pipe is raised to a height at which it is freely laterally movable, the lower end of the suction pipe having the suction mouth is, likewise, caused to move sideways due to the sloping condition of the suction pipe caused by the progresssive lateral movement, independent of the degree of such slope and the flow resistances prevailing at the suspended suction pipe, so that merely a predetermined period of time needs pass until the next work cycle is initiated, to ensure that the suction head has traversed the required distance. Consequently, notwithstanding the great lengths of the freely hanging suction pipe at a sufficiently controlled lateral movement of the floating body, it is possible to calculate with great precision the spacing of the depressions in the sea bottom sediment. Acoustic positioning means may be used to control the operation.
To assist the suction mouth in the penetration of the sediment, the invention provides for mechanical drilling, stripping or scraping means attached to the suction mouth, which means are rotated during the lowering and/or lifting of the suction mouth. Such rotational movement can be accomplished without difficulty by rotating the entire suspended suction pipe on the surface of the sea so that any special drive means for the suction mouth, as they are for instance required in the prior art vibratory suction heads, can be dispensed with, which represents a considerable advantage when working in great depths and, in addition, at high temperatures as is the case, for example, in the Red Sea.
The invention also provides for apparatus for carrying out the method. Such apparatus comprises a floating body from which the suction pipe is suspended which has attached to its lower end the suction head provided with means for loosening the sediment. In accordance with the invention, such loosening means are so constructed that they exert only a low degree of frictional resistance with respect to the sediment when lowered into it, while they exert a high degree of frictional resistance when they are extricated from the sediment. This particular type of construction takes into consideration the fact that as the suction mouth is penetrating into relatively solid sediment layers, there is danger of lateral deflection or buckling of the suspended suction pipe hitting the formation. For this reason, the frictional resistance is kept low during penetration into the sediment, whereby such deflections are avoided. Conversely, when the suction assembly is lifted, the large loosening forces may come into their own without disadvantage.
Loosening means which have the mentioned properties may be of a variety of types. They may for instance comprise a worm which is freely rotatably mounted at the lower end of the suction pipe and, as it is being lowered, drills into the sediment, while during the lifting stroke, forces are operative which prevent a rotation of the worm. Consequently, the sediment in the area surrounding the suction head is dislodged or loosened in large scale like fragments. However, the worm may also be fixedly mounted on the suction pipe, and the desired rotation of it may be brought about by a corresponding rotation of the upper end of the suction pipe on the surface of the sea.
Another form of construction that the loosening means may take is that of a folding anchor which will open up during lifting so that the surrounding sediment will be gripped and pulled up in large scales.
As the suction head and loosening tool assembly is being raised, generating a great amount of frictional resistance in accordance with the invention, an area of reduced pressure is produced below the loosening tools. This feature is utilized by a further development of the invention, in that the suction head is provided below the loosening means having high frictional resistance values, e.g. flukes or pivot plates, with nozzles pointing downwardly or preferably sideways. Such nozzles are in communication by way of a channel with entry openings which are located at a sufficient height above the means having large frictional resistance values, e.g. the flukes or pivot plates. The reduced pressure thus generated has the effect that water is caused to rush into this area, precipitating a flushing and loosening process which continues during the entire extent of the upward movement.
The sediment in its upper layers frequently has a viscosity which is amenable to pumping. Moreover, during working the sediment, a cloud of whirling sediment particles is produced immediately above the sediment surface. To aid in the flushing process going on in the region below the loosening means and to augment its effect, water is used by suction action from the mentioned cloud of sediment particles or from fluid layers of sediment. Arranging the suction openings at fixed locations on the suction pipe may entail the probability that they are too high up, resulting in the undesirable admission by suction of sediment-free water.
To remedy this situation, according to a further embodiment of the invention, the entry openings are spread out over an extended vertical stretch of the suction pipe. A vertically movable cover pipe is arranged over the suction pipe to cover this spread. The cover pipe is provided with means for its height adjustment. This height adjustment may be effected in the simplest case by a rope hanging from a spot above the surface of the sea. It is preferred, however, to have such height adjustment means comprise floating bodies, whereby the total buouancy of the cover pipe and floating bodies is so adjusted that the cover pipe is maintained floating in a fluid layer of predetermined density. This type of assembly is capable of accommodating varying heights as, for example, in the case of a funnel-shaped depression the depth of which is slowly increasing while the level of essentially sediment-free water is decreasing.
Another form of the means for height adjustment of the cover pipe consists of supporting surface areas for engagement by the side edges or rims of the hole or funnel formed in the sediment. As the funnel-shaped depression in the sediment is widened and deepened, the supporting surface areas will follow the changing configuration and, thus, effect a lowering of the height of the cover pipe.